I've got a radio that uses 8 "D" batteries. I'm thinking they are 1.5 volts which would be 12v total 8 x 1.5 = 12. If I'm thinking right, can I power this from a 12v car battery?

The radio has two rows of batteries one with 3 and one row with 5. The radio is plastic so I can't take it apart but I can get to the contact points of each row of batteries. Any advice on if this would work and how to wire it up?

Inspect the battery compartment. The D cells should be stacked button end (+) to flat end (-) to add the volts of each cell to 12V total. If there is more than one row you'll observe that each row is a different direction and each end but the very ends of the D cell stack are connected to keep the cells in series. The ends of the D cell stack are +/- 12V. Connect the (+) from the car batt to the D cell button end of the stack and the (-) end to the D cell flat end.

If you have a cigarette lighter plug you can get 12v from that too.

Be careful as the car battery can deliver enough current to melt wires / cause fires if they short. Fuze it if you can.

Mods: sorry if this falls under automotive mods. Handle as necessary.

Good luck! Be careful and stay dry. Are you sure that instead of cannibalizing the car you shouldn't be hitting the road?